I’ve found there are three main categories of firearms writers use in cheap thrillers. I want to note that all three can be done well or done badly, and that even them being chosen poorly is almost never a story-breaker on its own.
Category A: Generic
Common to people with little knowledge of the subject matter, Category A firearms tend to be fallbacks on the most generic, widely used, and widely known boomsticks. Stuff like “M-16s, AR-15s, AK-47s, Glocks”, and “RPGs”.
Done Well: Common weapons are common for a reason. In many, arguably even most cases, you don’t really need to know the exact details. Just “the guard had a Glock” or something along those lines can do in many cases. Or even less.
Done Poorly: When it’s clear the author wasn’t doing much research and just took what they heard. This is clear when it’s accompanied by an incorrect caliber or some other fairly obvious detail, ie, one thriller with a “.25 Glock”. Often this is a “brown M&M” (from an infamous Van Halen contract that had a request for a bowl of M&Ms but no brown ones to make sure the contractors were reading it closely) that shows something else is off.
Category B: Specific
This ranges from knowing the specific kind of what a certain country/organization uses to the kind of exact descriptions [certain obscure AR-15 variant by certain obscure company] in [certain obscure caliber] with [certain obscure accessories] firing [exact weight of the bullet].
Done Well: In many cases, it’s more accurate to have someone who, for whatever reason, doesn’t use the most common firearm. It can add legitimate flavor and be a “bowl without brown M&Ms” in a good way.
Done Poorly: Besides the inherent issues with overly detailed exposition, this can be jarring if its combined with bad research in regard to something else. That something can even be other weapons-I’ve found super gun-exposition and terrible detail in anything bigger than a belt-fed MG. It’s like a sports story where the car used to drive the characters to the stadium is overly detailed (“A 1999 Ford Crown Victoria LX with a 4.6 liter V8…”), but then once they get there, says “And then they watched the Yankees win the Stanley Cup with twenty dunks.”
Category C: Exotic
There’s a lot of overlap with the first two categories here, but I feel it’s worth mentioning. Basically, “exotic” weapons that are very big (Desert Eagles! .44s! .44 Desert Eagles!), operate on a very unconventional system (The infamous G-11), or both are a staple of classic action-adventure fiction.
Done Well: I don’t fault an author for wanting to throw in their favorite obscure “pieces”, I do the same in a lot of my CMANO scenarios with aircraft and ships, and especially if they know what they’re doing, it can be fun. Like knowing the impractically of a Desert Eagle but giving it to a Ziggy Sobotka-esque dummy as a sign of his style-over-substance personality, or knowing the legitimate advantages/capabilities of an exotic and using it.
Done Poorly: This can have the flaws of either category, amplified by the nature of the weapons themselves. The “common exotics” lean more to Category A, while ones the author has a specific liking to move more to Category B.